1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic recording medium suitable for a vertical recording system.
2. Description of the Prior Art
A vertical recording system is one of the excellent magnetic recording systems and has excellent short wavelength recording characteristics. In the vertical recording system, a vertically magnetizable recording medium with an axis of easy magnetization extending in the direction perpendicular to a film surface thereof is required. When a signal is recorded on this medium, residual magnetization is oriented in the direction perpendicular to the film surface. Therefore, when a signal of a shorter wavelength is recorded, a demagnetizing field in the medium is reduced, resulting in an excellent reproduction output.
A vertical recording medium having a magnetic layer formed by sputtering on a nonmagnetic substrate directly or through a soft magnetic thin film such as Permalloy has been widely employed. The magnetic layer contains Co and Cr as the major constituents and has an axis of easy magnetization in the direction perpendicular to the film surface. If this magnetic layer contains Cr of not more than about 30% by weight, the crystal system has a hexagonal close-packed structure and the C-axis can be vertically oriented on the film surface. Further, saturation magnetization can be decreased until an anisotropic magnetic field in the vertical direction becomes larger than a demagnetizing field. Therefore, a vertically magnetizable film is manufactured.
However, the rate at which the magnetic thin film is formed is low according to the sputtering method. Thus, the vertically magnetizable film can not be manufactured at low cost. However, in the vapor deposition method including a method for partially ionizing vaporized atoms from a vapor source such as the ion plating method, the present inventor has found that a Co-Cr vertically magnetizable film can be formed quickly at a rate of several 1,000 .ANG./sec. In the vapor deposition method, the substrate is moved along the circumferential surface of a cylindrical can to form a thin film thereon. Then, a vertically magnetizable tape is manufactured with high productivity.
In order to form a Co-Cr deposition film into a vertically magnetizable film, the C-axis of the hexagonal close-packed structure must orient vertically with respect to the film surface and the anisotropic magnetic field in the vertical direction must be larger than the demagnetizing field. For this purpose, it is found in an experiment that the angle of incidence of vaporized atoms must be controlled with reference to a certain limit in vapor deposition.